U.S. patent number 4,654,622 [Application Number 06/781,557] was granted by the patent office on 1987-03-31 for monolithic integrated dual mode ir/mm-wave focal plane sensor.
This patent grant is currently assigned to Honeywell Inc.. Invention is credited to Norman A. Foss, Paul W. Kruse, Jr., R. Andrew Wood.
United States Patent |
4,654,622 |
Foss , et al. |
March 31, 1987 |
Monolithic integrated dual mode IR/mm-wave focal plane sensor
Abstract
A monolithic integrated focal plane sensor array having elements
sensitive to IR radiation and elements sensitive to mm-wave
radiation. The sensor elements of the array sensitive to mm-wave
have microantennas coupled to the sensors.
Inventors: |
Foss; Norman A. (North Oaks,
MN), Kruse, Jr.; Paul W. (Edina, MN), Wood; R. Andrew
(Bloomington, MN) |
Assignee: |
Honeywell Inc. (Minneapolis,
MN)
|
Family
ID: |
25123131 |
Appl.
No.: |
06/781,557 |
Filed: |
September 30, 1985 |
Current U.S.
Class: |
338/14;
257/E27.131; 343/700MS |
Current CPC
Class: |
G01J
5/0837 (20130101); G01J 5/20 (20130101); H01L
27/14603 (20130101); H01Q 1/38 (20130101); H01Q
5/42 (20150115); H01Q 9/16 (20130101); H01Q
15/02 (20130101); H01Q 19/062 (20130101); H01Q
9/065 (20130101) |
Current International
Class: |
G01J
5/20 (20060101); H01Q 1/38 (20060101); H01L
27/146 (20060101); H01Q 9/16 (20060101); H01Q
15/02 (20060101); H01Q 9/04 (20060101); H01Q
19/06 (20060101); H01Q 15/00 (20060101); H01Q
9/06 (20060101); H01Q 5/00 (20060101); H01Q
19/00 (20060101); H01C 007/00 () |
Field of
Search: |
;338/14,22R,22SD,23,24,25 ;73/432AD ;357/51,60 ;343/7MS,720,721
;250/250,334 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
David B. Rutledge et al, "Imaging Antenna Arrays", IEEE
Transactions on Antennas & Propagation, vol. AP-30, No. 4, Jul.
1982, pp. 535-540. .
David Rutledge, "Integrated Circuits for Millimeter Waves", Nov.
'84, Cal. Inst. Technology-Engineering & Science, pp.
19-22..
|
Primary Examiner: Goldberg; E. A.
Assistant Examiner: Lateef; M. M.
Attorney, Agent or Firm: Dahle; Omund R.
Claims
The embodiments of the invention in which an exclusive property or
right is claimed are defined as follows:
1. An integrated dual-mode infrared and millimeter-wave focal plane
sensor array monolithically fabricated on the same silicon wafer
comprising:
a wafer of single crystal silicon having opposing flat
surfaces;
a first linear or two-dimensional array of infrared sensitive
sensors being fabricated on a first surface of said wafer said
infrared sensors each having electrical output means; and,
a further linear or two dimensional array of mm-wave sensitive
sensors fabricated on said first surface interspersed with said
first array sensors, said further array sensors being dimensionally
too small to effectively couple mm-wave energy, said mm-wave
sensors each having connected thereto a microantenna of a size
substantially matched to the wavelength of the mm-waves for
coupling received mm-wave radiation to the mm-wave sensors, said
mm-wave sensors each having electrical output means.
2. The article according to claim 1 in which the first array and
the further array are linear arrays.
3. The article according to claim 1 in which the first array and
the further array are two dimensional arrays.
4. The article according to claim 1 in which said microantennas are
bow-tie antennas.
5. The article according to claim 4 in which said bow-tie antennas
are fabricated on a second of said opposing flat surfaces and are
connected through the wafer to the mm-wave sensors.
6. The article according to claim 1 in which said microantennas are
full-wave dipole antennas.
7. The article according to claim 1 in which said infrared sensors
and said mm-wave sensors are bolometers.
8. The article according to claim 1 in which said microantennas are
fabricated to said wafer.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
A monolithic integrated focal plane sensitive to both mm-waves
(typically 94 GHz) and (typically 3-5 and 8-12 micron) IR radiation
is constructed on a silicon wafer by selective anisotropic etching
to fabricate microbolometer radiation sensors in a linear or
two-dimensional array. Sensors intended for IR detection are coated
with an IR absorbing material. Those intended for mm-wave sensing
are connected to metal film antennas deposited on the surface of
the silicon wafer. In this structure there is combined known
silicon IC processing techniques with a rugged high-g-load-tolerant
structure that permits the thermal condution losses to approach the
radiative losses of the element. Of particular importance is the
combining and interspersing of millimeter wave sensors with high
performance infrared sensors and electronics on the same silicon
chip, and fabricating in the same processing steps.
The fabrication of novel three-dimensional microelectronic devices
in a semiconductor crystal, typically silicon has been accomplished
by fabricating the device through many techniques including
isotropic and anisotropic etching. These techniques utilize the
cystalline structure of a single crystal semiconductor. An example
is the Johnson et al U.S. Pat. No. 4,472,239, "Method of Making
Semiconductor Device", assigned to the same assignee as the present
invention. The referenced patent shows that the technique is known
to manufacture micromechanical devices by etching into single
crystal silicon. The citation of this patent is provided merely as
background and is not deemed as prior art to the specific invention
claimed in this application.
In the prior art, such as U.S. Pat. No. 3,801,949, there has been
taught an infrared sensitive solid-state imaging device which is
small in size and which has a two-dimensional array of IR detector
elements in an integrated microcircuit. The detector array is
fabricated on a single crystal silicon substrate coated with a thin
layer of electrical insulating material, such as silicon dioxide or
silicon nitride. Etched openings are made in the silicon beneath
the insulating layer wherever a sensing element is desired for the
purpose of thermally isolating the sensing elements from their
surroundings. In the present invention an integrated dual-mode
IR/millimeter-wave sensor array is taught. The section of the
magnetic spectrum including millimeter waves and 3-5 or 8-12 micron
infrared radiation is shown in FIG. 1. The mm-waves of about 94 GHz
and the 3-12 micron IR are several orders of magnitude apart in
frequency and devices for sensing or detecting these two categories
differ substantially. It is desired to fabricate a monolithic
integrated two-dimensional focal plane array which has array
elements sensitive to 3-5 and/or 8-12 micron IR and elements
sensitive to mm-waves. The individual integrated sensors are about
0.1 mm in size and do not effectively couple the energy from the
mm-waves which are of a greater wavelength. It has been discovered
that when the integrated sensor elements intended for mm-wave
detection are provided with antennas (such as full wave dipoles or
bow-tie type) a successful mm-wave energy coupling apparatus is
achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a pertinent section of the electromagnetic
spectrum;
FIG. 2a is one embodiment of a microsensor linear array;
FIG. 2b discloses in two-dimensional geometry one embodiment of an
integrated dual-mode IR sensors and mm-wave sensors;
FIG. 3 shows a cross section of a microsensor structure;
FIG. 4 shows the front surface detail of full-wave dipole antenna
integrated IR/mm-wave array;
FIGS. 5a and 5b show detail of high thermal isolation
microsensor;
FIG. 6 shows the overall scanned array functional diagram;
FIGS. 7 and 8 show the dual mode sensor design using bow-tie
microantenna design.
FIG. 9 shows one embodiment of a dual-mode system illustrated
schematically in FIG. 6.
DESCRIPTION
Referring now to FIG. 2a there is shown a linear geometry version
of a monolithic integrated dual mode IR/mm-wave microsensor linear
array. Two-dimensional arrays mayb be obtained by constructing
several adjacent linear arrays. A focal plane sensitive to both IR
radiation (3-5 and/or 8-12 micron) and mm-waves is constructed on a
semiconductor substrate wafer 10, such as monocrystalline silicon.
The microsensors 12 intended for IR sensing are coated with an IR
absorbing material such as a thin metal film. The microsensors 13
intended for mm-wave sensing are connected to metal film antennas
14 deposited on the surface of the silicon wafer. A two-dimensional
geometry version is shown in FIG. 2b in which IR sensitive
microsensor arrays 15 are formed on one surface of the silicon
wafer 10 and antennas 16 are formed on the other surface of the
wafer. Thsi embodiment will be described later.
In FIG. 3 there is shown a cross section of a microsensor structure
showing the thermal isolation configuration as taught in U.S. Pat.
No. 4,472,239, above referenced. The microsensor imaging array is
on a silicon chip 19, based upon anisotropic silicon etching in
which a small mass, thin film radiation detector 20 is fabricated
into a thermally isolated dielectric cantilever structure 21 on the
surface of the silicon chip. The small mass and thermal isolation
provide arrays with excellent detector sensitivity and response
time. The millimeter-wave array uses planar dipole or bow-tie type
antennas to couple the mm-wave radiation to the thermally
integrating microsensors.
In FIG. 4 there is shown a detail of the front surface of a
full-wave dipole antenna type integrated infrared/millimeter wave
sensor electronically scanned linear array. Interspersed with the
multielement (ex.=300) IR detector elements 12' are a plurality
(ex.=10) of antenna coupled mm-wave elements 13'. Also shown in
block form is a bipolar pre-amp array 30 and an FET multiplexer 31.
An IR/mm-wave output signal is detected at 32. A partial cross
section of FIG. 4 cut through the detector array is shown in FIG.
5a. The silicon wafer 10 also includes a dielectric layer 33 and a
copper ground plane 34. A detail of one of the high thermal
isolation microsensors 35 is shown in the balloon of FIG. 5b in
which a resistor sensor 20 carrying dielectric cantilever 21 is
thermally isolated by the etch cavity in the silicon.
A structure which is required to couple efficiently to a mm-wave
radiation field must necessarily have dimensions of the order of
about the wavelength, e.g. 3 millimeters at 94 GHz). In the case of
an uncooled sensor, a sensitive area this large would lead to
degraded responsivity or response speed due to the increased
thermal mass of the sensor. We therefore require dimension of a few
mils, and must therefore couple the sensor to the radiation field
via an antenna structure with dimensions of the order of the
wavelength. Since microsensor arrays can be conveniently fabricated
on silicon substrates by photolithographic processes, we desire the
antenna and any coupling waveguides to be planar in design.
The mm-wave array portion, such as shown in FIGS. 2a and 4, is
further shown in FIG. 6 and consists of a silicon substrate 10',
upon which we use photolithography to fabricate an array of planar
microantennas 40, coupling waveguides 41 and microsensors 13, with
electrical leadouts to an electronic readout circuit as shown in
FIG. 6. MM-wave radiation is collected by the microantennas 40, and
coupled 41 to the dissipative load of the microsensors 13, whose
temperatures will rise causing the resistance to change. A
low-noise electronic circuit including a column address mux.42 and
a row address mux.43 monitors the resistances of the microsensor
elements and provides electrical signals 44 to output circuitry
dependent on the application such as target detection and
recognition.
The microsensor consists of a low-mass sensor element 20 which is
almost completely thermally isolated from its supporting structure
as shown earlier in FIG. 3. A resistance element is fabricated on
the sensor using a material whose resistance changes with
temperature. Any electrical power dissipated in this sensor
resistance (e.g. by direct infrared radiation on the sensor or by
mm-wave radiation coupled in from an antenna) heats the sensor
element 20 by an amount inversely proportional to the sensor
thermal mass and thermal conductance to the supporting structure.
The sensitivity of the microsensor requires a low thermal mass
sensor and good thermal isolation. The dissipated heat will flow to
the supporting structure with a time constant given by the sensor
thermal capacity times the thermal resistance to the surroundings.
This response time can be arranged to be milliseconds without
sacrificing sensitivity; faster response times can be achieved by
trading off sensitivity. The electrical output signals are obtained
by the use of a readout circuit which is sensitive to resistance
changes in the microsensor resistance.
The ultimate signal to noise ratio of such a microsensor is
achieved by the use of a very small sensor thermal mass, and very
high thermal isolation from the supporting structure. The minimum
noise level possible is due to Johnson noise in the sensor load
resistance, preamplifier noise and to fluctuations in the radiative
and conductive power interchanged between the sensor and its
surroundings. In the case of mm-wave radiation coupled electrically
into a microsensor from a microantenna, the sensor may be coated
with a highly reflective material so that radiation interchange
noise can be reduced to a low level. In this case the noise limits
would be due to (a) Johnson noise, (b) amplifier noise and (c)
thermal conduction noise.
Of particular importance is the very low conduction noise which is
achieved by the excellent thermal isolation and low mass of the
proposed structure. Using typical parameter values demonstrated by
the prototype devices, we calculate that noise equivalent power
levels of 6.times.10.sup.-12 watts/.sqroot.Hz are expected,
assuming 75% coupling efficiency to the radiative mm-wave field.
This calculated figure is in close agreement with experimental data
obtained on prototype devices.
EXPERIMENTAL RESULTS
Prototype devices have been connected to an electronic readout
circuit designed to display small resistance changes on an
oscilloscope. The sensors were installed in a metal chamber that
could be evacuated to vary the sensor thermal leak. Windows of ZnS
and glass were available to admit IR and mm-wave radiation into the
sensor chamber. A 10 Hz chopper was mounted in front of the sensor
window. A 1000.degree. K black body IR source was used to calibrate
the sensor with an IR intensity of 7.times.10.sup.-4 W/cm.sup.2. A
sensor response of about 100 mV was observed with the sensor at
atmospheric pressure, and about 400 mV with the sensor cell
evacuated. A 3.2 mm (94 GHz) CW oscillator source was used to
illuminate the sensor with a mm-wave intensity of about
2.times.10.sup.-3 w/cm.sup.2 at the sensor. The observed signal
amplitude from the sensor was measured at 280 mV. The mm-wave
signals increased in amplitude by about a factor of four as the
cell pressure was reduced from 760 to 0.5 torr, indicating that the
signal was due to the normal microsensor thermal response
mechanism.
MICROANTENNA CONSIDERATIONS
The properties of planar antennas lying on dielectric (e.g. Si,
Si.sub.3 N.sub.4, SiO.sub.2) surfaces are quite different from
antennas in homogeneous media. The principal differences are (1)
the polar diagram is always heavily biased towards the dielectric,
so that efficient collection of radiation is biased towards
radiation incident from the dielectric side, and (2) additional
peaks in the polar diagram may occur: some peaks are found along
the substrate surface plane, indicating coupling to substrate
surface waves which will lead to cross-talk between adjacent
antennas on that surface. Although the polar diagram of a planar
antenna on a dielectric substrate is heavily biased towards the
dielectric, this bias can be reversed by depositing a metallic
ground plane (e.g. 2000.degree. A copper) on the back surface of
the silicon substrate as shown in FIG. 5a, so that all radiation is
reflected towards the air side, and the antenna only "looks"
towards the air. This arrangement is very desirable, since IR
sensors receive radiation from the airside, and common reflective
optics can then be used for an array of mm-wave and IR-sensors
fabricated on the same silicon wafer.
An alternate modification alluded to earlier is the use of
"bow-tie" antenna designs where the incident radiation is through
the dielectric substrate. Our tests have shown that bow-tie
antennas can be used in linear arrays to efficiently collect
mm-wave radiation incident through the substrate. In this
configuration the IR radiation is absorbed in the front side
detector elements while the mm-wave radiation passes through the
silicon wafer and is collected by the backside bow-tie antennas
(FIG. 8). In this approach through-the-wafer interconnects from
antenna to sensor are preferably used. This alternate approach
offers good performance, with
Simple, planar geometry fabricated from metal films deposited on Si
wafer surfaces.
A polar diagram heavily biased (by a factor n.sup.3), where n is
the refractive index, into the dielectric, with beam width
tailorable by adjustment of the bow-tie angle as shown in FIG.
8.
A resistive characteristic impedance, tailorable by adjustment of
the bow-tie angle, constant over wide frequency range.
COUPLING OF ANTENNA TO MICROSENSORS
The simplest way of coupling a dipole antenna to a radiation sensor
is to fabricate the sensor between the arms of the dipole and
metallize the antenna to the sensor load. The antenna impedance can
be matched to sensor loads in the 100 ohm range.
* * * * *